1. Field of the Invention
The present invention relates to a method for driving a display medium, and relates more particularly to a display medium driving method for applying a voltage pulse to a display medium containing two types of particles of different color and charge characteristics.
2. Description of Related Art
Twisting ball displays (dichroic twisting ball displays), electrophoretic image displays, magnetophoretic image displays, thermal rewritable image displays, and liquid crystals capable of storing an image have been proposed as display media capable of repeating image display.
Of these display media, thermal rewritable image displays and liquid crystals capable of storing an image offer excellent image retention but suffer from low contrast between the image and non-image (background) areas because they cannot present a paper-white background. It is therefore difficult to present a sharp image.
Furthermore, display media using electrophoresis or magnetophoresis have color particles that can be moved by an electrical or magnetic field dispersed in a white fluid medium. Images are formed by making the color particles adhere to the display surface in the image area to display the color of the color particles while removing the color particles from the display surface in the non-image areas to display the white of the white fluid. Furthermore, these types of displays can store an image because the color particles do not move without an electrical or magnetic field being applied. However, while these displays can present a clear white background, sufficient image density cannot be achieved due to the intrusion of the white fluid into the gaps between color particles in the image areas. Sufficient contrast therefore cannot be achieved between the image and non-image areas. A sharp image display therefore cannot be achieved. Furthermore, if the display medium is bent when removed from the image display, there is the possibility of the white fluid leaking from the display medium.
Furthermore, twisting ball displays present an image by applying an electric field to spheric particles (balls), half of each ball being coated white and the other half black, to selectively reverse ball orientation. For example, the balls are driven to present the black side to the surface of the display in the image areas, and present the white side to the display surface in the non-image areas. This type of display can also store an image because the balls do not change orientation unless a field is applied. This type of display medium can also be manufactured in sheets relatively easily because the inside of the display medium contains substantially only solid particles, although oil is present only in the cavities around the particles. However, this display medium cannot present a 100% white display in principle even if a field is applied to the display medium so that the entire display surface shows white because ambient light rays penetrating the gaps between the balls are not reflected and are lost inside the display medium. In addition, light absorption and diffusion by the cavities means that only a gray tinged white display can be achieved. It is also difficult to completely reverse the particles, leading to a drop in contrast and, as a result, making it difficult to present clear images. Moreover, because particle size must be smaller than the pixel size, minute particles coated different colors must be manufactured in order to achieve a high resolution display. This requires sophisticated manufacturing technology.
A medium having a conductive color toner and white particles sealed between a pair of opposing substrates has also been proposed as an image display medium with a white background. With this type of image display medium a charge is injected to the conductive color toner through a charge transport layer disposed on the inside electrode surface of the back substrate, and the charge-injected conductive color toner is moved to and caused to adhere to the inside of the display substrate by applying a field between the electrode panels to display an image (see Japan Hardcopy '99, pp. 249-252). This image display medium consists of only solids, and color of pixels can, in principle, be changed completely. The problem with this type of display medium is that contrast cannot be sufficiently improved because there is conductive color toner not in contact with the charge transport layer displayed to the inside electrode surface of the back substrate, there is also conductive color toner independent of other conductive color toner, and these conductive color toners are found randomly dispersed between the substrates because a charge is not applied thereto and the toner particles therefore are not moved by an electric field.
[Problem to be Solved]
We have proposed (Japanese Patent Application 2000-165138) a display medium having a pair of substrates and plural types of particle groups of different color and charge characteristics sealed between the substrates. This display medium can also store images because the particles do not move unless a field is applied. Fluid leaks also do not occur because the display medium contains only solids. A high contrast, sharp image can also be displayed because a 100% change in color of pixels can, in principle, be achieved.
However, to achieve sufficient image density with this type of display medium by driving the charged color particles with an electric field, it is necessary to apply a high voltage, typically of several hundred volts or more although this varies with the substrate gap. This high voltage requirement produces the following problems.
First, discharge and electric leaks occur easily if the wiring density is high, and it is therefore difficult to achieve a high resolution display. Furthermore, the lack of semiconductor devices suitable for controlling such high voltages means that mechanical relays must be used for electrical switching in the drive circuit. This makes it difficult to reduce circuit size and lower cost. In addition, applying a high drive voltage increases particle velocity when the particles are driven, resulting in particles colliding with each other and with the substrates. This leads to change in the electrical properties of the particles and degradation of the substrate surface, shortening display medium life.
There are both passive matrix and active matrix display medium drive methods. If the display comprises an n pixel by n pixel matrix, an active matrix drive method driving each individual pixel requires (n×n, or n×n×2) signal paths while a passive matrix drive method driving each individual line requires only (n+n) signal paths. Compared with an active matrix drive method, passive matrix drive uses an extremely simple drive circuit and offers the benefit of greatly lowering cost.
Passive matrix drive is further described next. Passive matrix drive is used with a display medium having a display substrate and back substrate, each having formed thereon plural parallel line electrodes at equal intervals. The substrates are assembled so that the electrodes of the two substrates are mutually orthogonal as shown in FIG. 1. Note that for simplicity the display medium in FIG. 1 has a 4×4 passive matrix configuration with four column electrodes (a-d) on the display substrate and four row electrodes (i-iv) on the back substrate. The electrodes of both the display and back substrates are also connected to the power supply with a passive matrix drive. To form an image, all pixels are normally first driven to a uniform color and only the color of the desired pixels is changed. To change the image one line at a time, a voltage is applied sequentially one line at a time to the row electrodes of the back substrate according to a line write signal while synchronously applying a voltage to the desired column electrodes of the display substrate according to a pixel signal.
Consider, for example, changing the color of the pixels at row i columns a and c, row ii columns b and d, and row iii columns a and c, from black to white using passive matrix drive in a display medium containing positively charged black particles and negatively charged white particles with the entire display substrate displaying black (see FIG. 1). In this case voltage VLW is first applied only to row i while synchronously applying voltage VSW to columns a and c, thus changing the color of the pixels at row i column a and row i column c where both voltages are applied from black to white. Next, voltage VLW is applied to only row ii while synchronously applying voltage VSW to columns b and d, thus changing the color of the pixels at row ii column b and row ii column d from black to white. Then voltage VLW is applied to only row iii while synchronously applying voltage VSW to columns a and c, thus changing the color of the pixels at row iii column a and row iii column c from black to white.
The drive field changing a pixel from black to white is thus (VSW−VLW)/d where d is the substrate gap. Note that a field of VSW/d or VLW/d is also applied to the pixels that do not change color. It is therefore necessary for the display not to change, that is, for the particles not to move, when VSW/d or VLW/d is applied. At the same time (VSW−VLW)/d is preferably as high as possible to achieve high display contrast. VSW is therefore usually set to the voltage at which white particles begin to move (the voltage at which image density begins to change, referred to below as the “particle drive start voltage”), and VLW is set to the voltage reversing the polarity of the particle drive start voltage for white particles.
When color of pixels is changed from white to black with passive matrix drive, VSB is set to the particle drive start voltage of black particles, and VLB is set to the voltage reversing the polarity of the particle drive start voltage for black particles.
To accomplish passive matrix drive as described above, the applied voltage-image density curve of the display medium must rise sharply after the particle drive start voltage is exceeded as shown in FIG. 30, and sufficient density must also be achieved at twice the particle drive start voltage.
However, as also shown in FIG. 8, the particle drive start voltage is low and the change in image density due to the subsequently applied voltage is gradual with the above display medium, and the desired image density cannot be achieved without a high voltage. Sufficient image density therefore is not achieved at twice the particle drive start voltage, and passive matrix drive cannot display images with sufficient contrast.
The present invention was conceived with consideration for the aforementioned problems, and provides a drive method for a display medium containing two types of particles whereby driving at a low voltage is possible and, as a result, high image resolution, drive circuit downsizing, low display cost, and improved display durability can be achieved.
Our invention also provides a drive method for a display medium containing two types of particles whereby high contrast can be achieved with passive matrix drive, and significant cost reduction in the drive circuit can be achieved.